This invention relates to a method of preparing a catalyst containing Rh and/or Ru supported on a MgO carrier and to a process of producing a synthesis gas.
A synthesis gas which is a mixed gas containing hydrogen and carbon monoxide is widely used as a raw material for the synthesis of ammonia, methanol, acetic acid, etc.
Such a synthesis gas may be produced by reforming a hydrocarbon with steam and/or carbon dioxide in the presence of a catalyst such as Ni or other transition metals. In the reforming reaction, however, carbon deposition occurs as a result of side reactions to cause a problem of catalyst poisoning.
EP-A-0974551 discloses a Rh and/or Ru-supporting MgO catalyst which can decrease carbon deposition. Because noble metals such as Rh and Ru are very expensive, there is a great demand for a method which can produce noble metal-containing catalyst using as small an amount of the noble metal as possible.
In accordance with one aspect of the present invention there is provided a method of preparing a catalyst, which includes mixing magnesium oxide with a binder selected from the group consisting of carbon, fatty acids having 12-22 carbon atoms, magnesium salts of fatty acids having 12-22 carbon atoms, carboxymethyl cellulose, a magnesium salt of carboxymethyl cellulose, and polyvinyl alcohol to obtain a mixture in which the binder is present in an amount of 0.1-5% by weight. The mixture is molded and calcined at a temperature of at least 1,000xc2x0 C. to obtain a carrier having a specific surface area of 5 m2/g of less. The carrier is impregnated with an aqueous solution containing a catalytic metal component selected from the group consisting of rhodium compounds, ruthenium compounds and mixtures thereof so that the catalytic metal component is loaded on the carrier in an amount of 10-5,000 ppm, in terms of elemental metal, based on the weight of the carrier, and then dried and calcined to give a catalyst.
In another aspect, the present invention provides a process for the production of a synthesis gas, wherein a carbon-containing organic compound is reacted with steam and carbon dioxide at a temperature 600-1,000xc2x0 C., a pressure of 5-40 Kg/cm2 G and a GHSV of 1,000-10,000 Hrxe2x88x921 in the presence of a catalyst, obtained by the above method, with a molar ratio of the steam to carbon of the carbon-containing organic compound of 2 or less and a molar ratio of the steam to the carbon dioxide of 0.1-10.
The present invention further provides a process for the production of carbon monoxide, wherein a carbon-containing organic compound is reacted with carbon dioxide at a temperature 600-1,000xc2x0 C., a pressure of 5-40 Kg/cm2G and a GHSV of 1,000-10,000 Hrxe2x88x921 in the presence of a catalyst, obtained by the above method, with a molar ratio of the carbon dioxide to carbon of the carbon-containing organic compound of 1.0-3.0 to obtain a synthesis gas, and wherein the synthesis gas is treated to concentrate carbon monoxide.
In the method of the present invention, a carrier of MgO (magnesium oxide) having a specific surface area (SA) of 5 m2/g or less is used. Thus, in the first step, magnesium oxide is mixed uniformly with a binder selected from carbon, fatty acids having 12-22 carbon atoms, magnesium salts of fatty acids having 12-22 carbon atoms, carboxymethyl cellulose, a magnesium salt of carboxymethyl cellulose, and polyvinyl alcohol to obtain a mixture. The binder serves as a molding aid for molding magnesium oxide into a desired shape and as a lubricant which can facilitate the molding operation because of easiness in filling or charging the mixture to a molding device. The use of carbon as the binder is preferred for reasons of its availability at low costs. The carbon may be, for example, graphite, carbon black or activated carbon. Examples of the fatty acids include lauric acid, myristic acid, palmitic acid, stearic acid and behenic acid.
Each of the magnesium oxide and binder is preferably in the form of powder having an average particle diameter of 1-1,000 xcexcm, more preferably 10-100 xcexcm. The binder is used in an amount of 0.1-5% by weight, preferably 2-4% by weight, based on the total weight of the magnesium oxide and the binder, namely the mixture. When the amount of the binder is smaller than 0.1% by weight, a molded product obtained by molding the mixture as described hereinafter and a carrier obtained by calcining the molded product as described hereinafter fail to show satisfactory mechanical strengths. Additionally, such a small amount of the binder cannot improve easiness in filling or charging the mixture to a molding device. Too large an amount of the binder in excess of 5% by weight is undesirable because the binder retains in the carrier after calcination, causing reduction of mechanical strengths of the carrier. The use of an excess amount of the binder is also disadvantageous from the standpoint of economy.
The mixture is then molded into a molding device. The molding operation is generally performed at a room temperature and a pressure of 100-3,000 kg/cm2G, preferably 200-2,000 kg/cm2G, using any suitable molding method such as a press molding method or a tablet making method, to produce a molded product having any desired shape such as in the form of tablet, rod, ring or cylinder. The molded product generally has a size (diameter or longitudinal length) of 3-30 mm, preferably, 5-25 mm. The shape and the size of the molded product are suitably determined in view of the kind of catalyst bed used.
The molded product is then calcined at a temperature of at least 1,000xc2x0 C., preferably 1,100-1,300xc2x0 C., to obtain a carrier having a specific surface area of 5 m2/g of less. Upper limit of the calcination temperature is generally 1,500xc2x0 C. Magnesium oxide having a large specific surface area may be used as a raw material in the present invention, because sintering of magnesium oxide proceeds significantly during the calcination at 1,000xc2x0 C. or more so that the specific surface area of the magnesium oxide carrier obtained can be reduced to 5 m2/g of less. The calcination is carried out in an oxygen-containing atmosphere, generally air, for at least 1 hour, preferably at least 3 hours. The upper limit of the calcination time is not specifically limited, but is generally about 72 hours. During the course of the calcination, the binder contained in the molded product is decomposed and disappears. The carrier thus obtained has high mechanical strengths of, for example, a side crashing strength of 30-70 kg/piece.
The thus obtained MgO carrier having a specific surface area of 5 m2/g or less has a high degree of crystallinity and has stable surfaces having reduced strong acid sites. Namely, the MgO thus stabilized has a Hammett acidity function (Ho) of 2 or more and has an amount of the acid sites of not greater than 0.03 mmol/g.
When the specific surface area of the MgO carrier is greater than 5 m2/g, the degree of crystallinity becomes low and the amount of catalytic metal (Rh or Ru) supported thereon is unavoidably increased. Additionally, the acid strength is so increased that the resulting catalyst may cause undesirable reactions resulting in deposition of carbon on the catalyst. When the specific surface area of the MgO carrier is extremely low, the amount of the catalytic metal supported thereon is very small. At least 0.01 m2/g is desirable to obtain satisfactory catalytic activity. Preferably, the specific surface area of the MgO carrier is 0.05-3 m2/g.
The term xe2x80x9cspecific surface areaxe2x80x9d referred to in the present specification in connection with catalyst or MgO carrier is measured by the xe2x80x9cBET methodxe2x80x9d at a temperature of 15xc2x0 C. using a measuring device xe2x80x9cSA-100xe2x80x9d manufactured by Shibata Science Inc.
The catalytic metal which is at least one of Rh and Ru may be loaded on the MgO carrier by any known method such as impregnation by spraying or immersion or ion exchange using an aqueous solution containing a water soluble compound, such as halide, nitrate, sulfate, organic acid salt (e.g. acetate) or complex (chelate compound) of rhodium and/or ruthenium.
The amount of the catalytic metal (Rh and/or Ru) loaded on the MgO carrier is 10-5,000 ppm, preferably 100-2,000 ppm, in terms of elemental metal, based on the weight of the MgO carrier. An amount of the catalytic metal above 5,000 ppm is undesirable because the costs of the catalyst increase and because carbon deposition occurs. Too small an amount of the catalytic metal below 10 ppm fails to provide satisfactory catalytic activity. The amount of the catalytic metal may be controlled by control of the specific surface area of the MgO carrier or by control of the conditions under which the catalytic metal is loaded on the MgO carrier, such as concentration of the catalytic metal compound in the aqueous solution.
The MgO carrier to which the catalytic metal has been loaded is then dried at any suitable temperature, preferably 30-150xc2x0 C. The dried carrier is subsequently calcined at a temperature of preferably at least 200xc2x0 C. for at least 1 hour. The upper limit of the calcination temperature is not specifically limited but is generally about 1,100xc2x0 C. The calcination temperature is preferably at least 2 hours, more preferably at least 3 hours. The upper limit of the calcination time is not specifically limited but is generally about 24 hours. As a consequence of the calcination, the catalytic metal has improved thermal stability and, hence, the catalyst has excellent thermal stability.
The thus obtained catalyst has an amount of the catalytic metal of 10-5,000 ppm, preferably 100-2,000 ppm, based on the weight of the MgO carrier and a specific surface area of 0.01-5 m2/g, preferably 0.05-3 m2/g. Because the catalytic metal is prevented from aggregating but is in the form of fine particles uniformly dispersed on the MgO carrier having a high crystallinity and a small surface area, the catalyst exhibits satisfactory reforming activity while preventing carbon deposition, even though the amount of the catalytic metal is very small.
The thus obtained catalyst also has a Hammett acidity function (Ho) of more than 2, preferably at least 3.3 and has an amount of the acid sites of not greater than 0.03 mmol/g, preferably not grater than 0.02 mmol/g and, thus, the carbon deposition activity thereof is highly suppressed. The Ho value of the catalyst may be controlled by the calcination temperature and time of the molded MgO for the preparation of the MgO carrier.
The Hammett acidity function Ho is defined as the ability of the solid surface to convert an adsorbed alkaline base Bxe2x88x92 into its acid form Bxe2x88x92H+ and is represented by the following formula:
Ho=pKa (=pKBxe2x88x92H+)+log[B]/[Bxe2x88x92H+]
wherein KBxe2x88x92H+ represents a dissociation constant of the acid form Bxe2x88x92H+ formed by a reaction between the basic indicator Bxe2x88x92 and the acidic site H+B, and [B]/[Bxe2x88x92H+] is a ratio of the concentration of the indicator present in the medium as a neutral base B to the concentration present as the acid form H+B. Stronger the acid site, the more is the tendency of the catalyst to transfer proton to the indicator and, therefore, Ho becomes smaller.
The Hammett acidity function Ho in the present specification is measured by xe2x80x9cBenesi methodxe2x80x9d disclosed in Handbook for Catalyst Experiments, p. 172, Kodansha Scientific, 1986. Discoloration of an indicator, whose pKa is known, upon being contacted with a catalyst indicates that the catalyst contains acid points whose Ho is smaller than the pKa of the indicator. Various predetermined amounts of butylamine which is a base are adsorbed on the acid points and titration is then performed using different indicators having different pKa. From the amount of the butylamine, the amount of the acid points can be determined. From the pKa values, the Ho value can be determined. Measurement is performed at room temperature.
The production of a synthesis gas may be performed by reacting a hydrocarbon feed with steam and/or carbon dioxide (CO2) in the presence of a catalyst prepared by the above-described method. As the hydrocarbon feed, a lower hydrocarbon such as methane, ethane, propane, butane or naphtha may be used. The use of methane is preferred. In the present invention, a natural gas (methane gas) containing carbon dioxide is advantageously used.
In the case of a method of reacting methane with carbon dioxide (CO2) (reforming with CO2), the reaction is as follows:
xe2x80x83CH4+CO2⇄2H2+2COxe2x80x83xe2x80x83(1)
In the case of a method of reacting methane with steam (reforming with steam), the reaction is as follows:
CH4+H2O⇄3H2+COxe2x80x83xe2x80x83(2)
In the reforming with CO2, the reaction temperature is 500-1,200xc2x0 C., preferably 600-1,000xc2x0 C. and the reaction pressure is an elevated pressure of 5-40 kg/cm2G, preferably 5-30 kg/cm2G. When the reaction is performed with a fixed bed system, the gas space velocity (GHSV) is 1,000-10,000 hrxe2x88x921, preferably 2,000-8,000 hrxe2x88x921. The amount of CO2 relative to the hydrocarbon feed is 20-0.5 mole, preferably 10-1 mole, per mole of carbon of the raw material compound.
In the reforming with steam, the reaction temperature is 600-1,200xc2x0 C., preferably 600-1,000xc2x0 C. and the reaction pressure is an elevated pressure of 5-40 kg/cm2G, preferably 5-30 kg/cm2G. When the reaction is performed with a fixed bed system, the gas space velocity (GHSV) is 1,000-10,000 hrxe2x88x921, preferably 2,000-8,000 hrxe2x88x921. The amount of steam relative to the hydrocarbon feed is 0.5-5 moles, preferably 1-2 moles, more preferably 1-1.5 moles, per mole of carbon of the raw material compound.
In the reforming with steam according to the present invention, it is possible to produce a synthesis gas in an industrially favorable manner while suppressing the carbon deposition, even when the amount of steam (H2O) is maintained 2 moles or less per mole of carbon of the hydrocarbon feed. In view of the fact that 2-5 moles of steam ratio is required in the conventional method, the present invention, which can permit the reforming reaction to smoothly proceed with an amount of steam of 2 moles or less, has a great industrial merit.
In a case where a synthesis gas is produced in the present invention by reacting a hydrocarbon feed with a mixture of steam and CO2, the mixing proportion of steam and CO2 is not specifically limited but is generally such as to provide a H2O/CO2 molar ratio of 0.1-10.
A process for the production of carbon monoxide includes, as a first step, a synthesis gas producing step. The first step is carried out by reacting a carbon-containing organic compound with carbon dioxide in the presence of a catalyst prepared by the method according to the present invention. In the reaction of the carbon-containing organic compound with carbon dioxide (reforming with CO2), the reaction temperature is 500-1,200xc2x0 C., preferably 600-1,000xc2x0 C. and the reaction pressure is an elevated pressure of 5-40 kg/cm2G, preferably 5-30 kg/cm2G. When the reaction is performed with a fixed bed system, the gas space velocity (GHSV) is 1,000-10,000 hrxe2x88x921, preferably 2,000-8,000 hrxe2x88x921. The amount of carbon dioxide relative to the raw material carbon-containing organic compound is 1-10 moles, preferably 1-5 moles, more preferably 1-3 moles, per mole of carbon of the raw material compound.
In the reforming with CO2 according to the present invention, a synthesis gas can be produced in an industrially advantageous manner while preventing carbon deposition, even when the amount of CO2 is maintained no more than 3 moles per mole of carbon of the raw material compound.
As a result of the above-described reforming with CO2, a synthesis gas containing hydrogen and carbon monoxide is obtained. When methane is used as a raw material, the synthesis gas has, for example, a composition containing 10-30 vol % of H2, 35-45 vol % of CO, 5-40 vol % of unreacted CO2, 0-30 vol % of unreacted CH4 and 5-20 vol % of H2O.
In the second step, the thus obtained synthesis gas is used as a raw material and carbon monoxide (CO) is concentrated therefrom. The CO concentration may be carried out by a customarily employed CO concentration method such as a cryogenic separation and a absorption method using an aqueous copper salt solution as an absorbent.
The above processes may be carried out with various catalyst systems such as a fixed (packed) bed system, a fluidized bed system, a suspension bed system and a moving bed system. The fixed bed system is preferably used.